  <nav class="navbar navbar-expand navbar-dark bg-dark fixed-top">
    <a class="navbar-brand" href="#">XXX管理系统</a>
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="lists.php">用户管理</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="login.php">用户登录</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="logout.php">退出登录</a>
      </li>
    </ul>
  </nav>